Tabular silver halide grains having a parallel twin plane (which hereinafter are referred to as "tabular grains") have the following photographic properties:
(1) The ratio of the surface area to the volume (hereinafter, the ratio is referred to as "specific surface area") of the tabular grains is large and a large amount of sensitizing dye(s) can be adsorbed onto the surface, whereby the spectral sensitivity is relatively higher than the inherent sensitivity thereof.
(2) When a silver halide emulsion containing the tabular grains is coated and dried, the grains are disposed in parallel to the surface of the support, whereby the thickness of the coated emulsion layer can be thinned to improve the sharpness of images.
(3) In an X-ray photographic system, by adding sensitizing dye(s) to tabular grain silver halide emulsions, the amount of crossover light can be remarkably reduced, since the extinction coefficient of the dye(s) is larger than the extinction coefficient by the indirect transition of the silver halide (AgX), whereby the degradation of image qualities can be prevented.
(4) The silver halide emulsion containing the tabular grains causes less scattering of light and hence can give images having high resolution.
(5) Since the specific surface area is large, the developing rate is high.
Owing to these various advantages, the tabular grain silver halide emulsions have hitherto been used for high speed photographic materials on market.
Tabular grains, of silver halide emulsions, having an aspect ratio of at least 8 (i.e., at least 8/1) are disclosed in JP-A-58-113926, JP-A-58-113927, JP-A-58-113928 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"), etc.
The term "aspect ratio" is the ratio of the diameter of the tabular grain to the thickness thereof, and is expressed herein as ratio of two numbers (if omitted, the second number is considered as being 1). Furthermore, the "diameter" of the grain is defined as the diameter of a circle having an area equal to the projected area of the grain as viewed in a micrograph or an electron micrograph of an emulsion. Also, the thickness of the grain is shown by the distance between two basal planes constituting the tabular grain.
However, as shown in the examples of the aforesaid patent, the tabular grains prepared by the conventional process show poor monodispersibility and hence have disadvantages such as that:
(1) the increase of contrast (i.e., high gamma) of the characteristic curve cannot be expected, and
(2) when the emulsion containing the large silver halide grains and the small silver halide grains is chemically sensitized, it is difficult to apply the optimum chemical sensitization to both of the silver halide grains, because the optimum condition for the chemical sensitization differs between the large grains and the small grains.
Accordingly, various attempts for improving the monodispersibility of tabular grains have hitherto been proposed.
In the monodisperse tabular grains disclosed in JP-A-52-153428 and U.S. Pat. Nos. 4,150,994, 4,184,877 and 4,184,878, there is a restriction of using AgI crystals as the nuclei and the content of the substantially circular tabular silver halide grains having two parallel twin planes, in the present invention (which grains are tabular grains having two parallel twin planes which are parallel also to the basal plane in one grain, and hereinafter are referred to as "circular tabular grains") in the grains obtained is less.
JP-A-142329 and U.S. Pat. No. 4,301,241 disclose the growing condition for monodisperse tabular grains, but since the nucleus forming condition for seed crystals used in the example was improper, the tabular grains formed contained less tabular grains having two parallel twin planes than in the case of the present invention.
JP-A-61-112142 discloses monodisperse twin grains, but since spherical grains are used as the seed crystals, the aspect ratio of the silver halide grains obtained in the examples thereof is up to 2.2, which is a low aspect ratio. Further, JP-A-61-112142 does not mention the control of supersaturation upon forming nuclei for the monodisperse twin grains.
JP-A-51-39027 discloses the same grain-forming process as the patent publication described above. However, the tabular grains in this patent literature contain no substantially the circular tabular grains.
Monodisperse circular tabular grains described in the examples of French Patent No. 2534036 show that at least 50% of the projected area are the one originating from the trigonal tabular grains according to the grain photograph shown there. As stated in J. E. Maskasky, Journal of Image Science, vol. 31, p.15-26, (1987), each of the trigonal tabular grains has triple parallel twin planes, which are different from the grains according to the present invention.
Since the twin planes are two dimensional defects which break the periodic lattice of crystals there, they hinder the transfer of photoelectrons or trap electrons and therefore increase the internal sensitivity. Accordingly, while it is necessary that a grain has two parallel twin planes for preparing the tabular grain, three or more twin planes are disadvantageous since the synergism effect with the increase of the number of planes increases the internal traps, makes the internal sensitivity higher and promotes the dispersion of latent images.
Accordingly, tabular grains as described above having three parallel twin planes per grain are not desirable since their characteristics are inferior to tabular grains having two parallel twin planes.
Japanese Patent Application No. 61-48950 of the present inventor et al., discloses a method of preparing monodisperse tabular grains.
Japanese Patent Application No. 61-299155 of the present inventor et al discloses monodispersed hexagonal tabular silver halide grains, and claims the grain form which is obtained mainly by the hexagonal edge dissolution of the monodisperse hexagonal tabular grains. Since, these hexagonal tabular grains have sharp edges, they involve the following problems: (1) The edge portions of the tabular grains having sharp edges are easily soluble and tend to be rounded with the lapse of time. That is, they tend to be rounded during the step of water washing or re-dispersion, during chemical ripening, etc. This change is remarkable, particularly in tabular grains of high aspect ratio or tabular grains of small size.
Although the aging deformation of the tabular grains can be prevented to some extent by previously adding additives such as sensitizing dyes and adsorbing them to the grains after the formation of the grains, it still remains as a problem. Particularly, when the chemically sensitized tabular grains are dissolved in the subsequent steps, it is not desirable since the chemically sensitized nuclei on the dissolvable portion are also affected together.
(2) For preparing silver halide grains of high sensitivity, it is usually necessary to restrict the position of the chemically sensitized nuclei in order to prevent the dispersion of latent images. In this case, a method is known of adsorbing additives preferentially adsorbed to the main surface of silver halide grains (for example, sensitizing dyes, anti-fogging agents or stabilizers alone or in admixture) and then adding a chemical sensitizer, thereby forming chemically sensitized nuclei in a portion not adsorbed by the additives. However, in the case of tabular grains, since both of the parallel main plane surfaces and the outer surfaces of the edge portions are (111) planes, it is difficult to prepare the chemically sensitized nuclei limited to the planes of the plane index different from each other, as stated in J. F. Hamilton and L. E. Braddy, Journal of Applied Physics, vol. 35, pp. 414-421 (1964).
Accordingly, tabular silver halide grains showing satisfactory monodispersibility, having two parallel twin planes, showing good aging stability and in which the position for the chemically sensitized nuclei is restricted (and accordingly, dispersion of latent images is inhibited) are desired as photographic silver halide grains.